Protesters broke through security and police cordons at the SkyCity Grand Hotel, while next-door John Key announced sweeping changes to employment law at the National Party's annual conference.

About 300 protesters gathered outside the Sky Tower before marching down the road. Former MP Sue Bradford - holding a National Distribution Union flag - broke through a security cordon with about 40 protesters.

They entered the hotel and shouted slogans against the Government's changes to employment law which include making it easier for employers to fire workers, extending the 90 day trial for new workers, and tightening union access to workplaces. Unions will need the consent of employers, which cannot be unreasonably withheld.

Protesters retreated from the hotel as security staff and police scrambled to stop them entering lifts.

The protest then shifted its focus to the door of the convention centre that was guarded by a strong police presence.

Despite protesters repeatedly charging the police line, shift commander Wayne Kitcher said there were no arrests or injuries to police officers.

Ms Bradford told nzherald.co.nz that she had been "belted in the face" by police. "I'm sure I will end up with a few bruises."

She said she was pleased with the number of union members who had turned out to oppose the Government's reforms that she likened to those under former Finance Minister Bill Birch in the early 1990s.

"It is important to show the National Party conference and John Key that this is an attack on working people," Ms Bradford said.

"There was a large number who wanted to get into the conference, but obviously the police stopped us getting in," she said.

Unite Union leader Matt McCarten said Mr Key's comments amounted to a declaration of war on working people.

He said Mr Key has enjoyed the support of some working people but now "the mask has come down".

"This country has changed for the worse and my advice to [Mr Key] would be to back down," Mr McCarten said.

Asked if the changes were necessary to stimulate job growth in a tough financial climate, Mr McCarten said that was "bullsh**".

"Employers do not employ people out of charity. They run a business and they want a job done," Mr McCarten said.

He said the Government's changes were about restoring power to employers and they amounted to a return to the Feudal System.

"They want to make us slaves in our own country and they know where they can stick that," Mr McCarten said.

Members from Unite, National Dairy Workers, National Distribution Union, Engineers, Printing and Manufacturing Union, and the New Zealand Educational Institute, amongst others turned out to listen to union leaders speak beneath an inflated giant rat.

Veteran protester John Minto said the rat had a "remarkable resemblance" to Mr Key. He told the crowd that 400,000 workers will either enter the work force for the first time or change jobs and all will be affected by the Government's changes.

Maxine Gay from the National Distribution Union said the issue was about "who owns the workplace".

She said denying workers the right to choose who to invite into their workplace "amounts to slavery".

Helen Kelly, National President of the Council of Trade Unions, described the Government's plans as a disgrace, she said 22 per cent of those employed under the existing 90-day scheme were dismissed and

47 per cent of those were people under the age of 23.

She said one of the underlying reasons for their dismissals' was because of their "attitude", she said that could have been because they joined a union or raised safety concerns.

Ms Kelly said she had seen many of these young people in her offices and the "impact was devastating".

"They have no idea what happened."

Unions are planning on meeting on Thursday to propose their next move.

Mr McCarten and Ms Bradford would not be drawn on what unions were planning.